<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .container {
            outline: 1px solid red;
            position: relative;
        }
        .container .item {
            width: 100px;
            height: 100px;
            text-align: center;
            line-height: 100px;
            margin: 16px;
            background-color: red;
        }

        .container .mask {
            width: 400px;
            height: 300px;
            background-color: royalblue;
            position: absolute;
            top: 0;
            left: 130px;
            display: none;
        }

        .container .item:hover~.mask {
            display: block;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="item">1</div>
        <div class="item">2</div>
        <div class="item">3</div>
        <div class="mask"></div>
    </div>
</body>
</html>